%%
%% The LaTeX Companion, 3ed
%%
%% Example 3-3-37 on page I-176 in "Customizing units and quantities".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3exa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{111.0pt}

%StartShownPreambleCommands
\usepackage{siunitx}
\DeclareSIUnit[per-mode=symbol]\kmh{\kilo\metre\per\hour}
%StopShownPreambleCommands

\begin{document}
You can drive \qty{50}{\kmh} in most cities.
\end{document}
